1. Field of the Invention
The present invention relates to a spin valve read head that has a high magnetic moment, high coercivity pinning layer, and more particularly to a pinning layer that pins a pinned layer with its high coercivity and does not significantly impact the coercivity of a free layer because of its low magnetic moment.
2. Description of the Related Art
A read head employing a spin valve sensor (hereinafter referred to as a "spin valve read head") may be combined with an inductive write head to form a combined magnetic head. In a magnetic disk drive an air bearing surface (ABS) of the combined magnetic head is supported adjacent a rotating disk to write information on or read information from a surface of the disk. In a write mode, information is written to the surface by magnetic fields that fringe across a gap between first and second pole pieces of the write head. In a read mode, the resistance of the spin valve sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals.
A spin valve sensor includes a nonmagnetic conductive layer, also called a spacer layer, sandwiched between first and second ferromagnetic layers, referred to as a pinned layer, and a free layer. First and second leads are connected to the spin valve sensor for conducting the sense current therethrough. The magnetization of the pinned layer is maintained ("pinned") at 90.degree. to the magnetization of the free layer. The magnetization of the free layer changes freely in response to magnetic fields from the rotating disk. The magnetization of the pinned layer is, typically, pinned by exchange coupling with an antiferromagnetic layer.
Preferably, the thickness of the spacer layer is less than the mean free path of conduction electrons through the spin valve sensor. With this arrangement, some of the conduction electrons are scattered by the interfaces of the spacer layer with the pinned and free layers. When the directions of magnetization of the pinned and free layers are parallel, scattering is minimal, and when the directions are antiparallel, scattering is maximized. Changes in the scattering change the resistance of the spin valve sensor in proportion to sin .theta., where .theta. is the angle between the magnetizations of the pinned and free layers. A spin valve sensor has a magnetoresistive (MR) coefficient that is preferably substantially higher than the MR coefficient of an anisotropic magnetoresistive (AMR) sensor. For this reason it is sometimes referred to as a giant magnetoresistive (GMR) sensor.
A spin valve transfer curve that plots the readback signal of the spin valve head versus the applied signal from the magnetic disk is defined by a substantially linear portion of sin .theta.. With positive and negative magnetic fields from a moving magnetic disk, which are equal in magnitude, it is important that positive and negative changes in the GMR of the spin valve read head be equal in order that the positive and negative readback signals be equal. When, in a quiescent state, the direction of the magnetic moment of the free layer is parallel to the ABS and the direction of the magnetic moment of the pinned layer is perpendicular to the ABS, the positive and negative readback signals will be equal when sensing positive and negative fields from the magnetic disk. When the direction of the magnetic moment of the free layer is parallel to the ABS, the bias point on the transfer curve will be zero, and the readback signals will be symmetrical about this bias point.
The antiferromagnetic layer interfacially engages the pinned layer in order to pin the magnetization of the pinned layer in a predetermined direction by magnetic exchange coupling. During construction of the magnetic head, the antiferromagnetic pinning layer is subjected to a temperature above a blocking temperature in presence of a magnetic field. The blocking temperature is the temperature at which the antiferromagnetic effect of the material ceases and the magnetic spins are free to align in the direction of the aforementioned magnetic field. When the temperature is reduced to ambient and the magnetic field is removed, the magnetic spins of the antiferromagnetic pinning layer are oriented in a predetermined direction that is, typically, perpendicular to the ABS. The antiferromagnetic pinning layer has a face that directly engages a face of the pinned layer. The orientation of the spins at the face of the antiferromagnetic layer orient the spins of the face of pinned layer in a like direction by exchange coupling. Consequently, all of the spins of the pinned layer are oriented in the predetermined direction. A high level of exchange coupling promotes high thermal stability of the head.
Since the antiferromagnetic pinning layer is not magnetized, it exerts no magnetic influence on the free layer. This is advantageous since the magnetization of the free layer should be free to rotate about a bias point in response to magnetic fields from the rotating disk. Advantageously, the magnetization of the pinned layer can be strongly pinned by the antiferromagnetic pinning layer so that its orientation cannot be easily changed by stray magnetic fields.
Unfortunately, a serious disadvantage of the antiferromagnetic pinning layer is that it loses its orientation when its blocking temperature is exceeded. As a consequence, the magnetic moment of the pinned layer is no longer pinned in the desired direction. This can be caused during construction of the head by electrostatic discharge (ESD) from an object carrying a static charge. More often, however, the blocking temperature is exceeded during use when the head frictionally engages an asperity on the magnetic disk. Accordingly, there is a strong-felt need to improve the thermal stability of a spin valve read head by improving the performance of the pinning layer.